Not sure why they do it. As far as indexing, this is what Google says about it:
"IFrames are sometimes used to display content on web pages. Content displayed via iFrames may not be indexed and available to appear in Google’s search results. We recommend that you avoid the use of iFrames to display content. If you do include iFrames, make sure to provide additional text-based links to the content they display, so that Googlebot can crawl and index this content."